1. Field of the Invention
This invention relates to carbamates and is more particularly concerned with an improved process for the preparation of carbamates from aromatic substituted ureas and organic carbonates.
2. Description of the Prior Art
The preparation of alkyl N-hydrocarbylcarbamates from N-hydrocarbyl substituted ureas and dialkyl carbonates using various types of basic catalysts has been disclosed in U.S. Pat. No. 3,627,813. The catalysts included alkali metal hydroxides, alkali metal alkoxides, and tertiary organic amines. When using the above method with ureas having other base sensitive chemical linkages, the strong basic catalysts can have a deleterious effect on the yield of desired carbamates.
U.S. Pat. No. 4,381,404 discloses a similar method to that disclosed in the patent cited supra. However, the catalysts employed are inorganic or organic compounds of aluminum, lead, magnesium, titanium, tin, zinc, or zirconium. While this reference avoids the strong bases of the prior patent, generally speaking, relatively high reaction temperatures are required in order to isolate the carbamate products in good yield.
It has now been found that aluminum metal in combination with traces of a promoter combination will catalyze the reaction of N-aromatic substituted ureas with organic carbonates to form the corresponding carbamates in high conversions.
Compared with the prior art methods, the process in accordance with the present invention provides an improvement because it avoids the use of strong basic catalysts and the need for high reaction temperatures.
Further, low cost aluminum in any form can be employed along with only trace amounts of the promoter combination. Accordingly, the present method provides additional economic benefits compared to prior art methods.